JP6682826B2 - Optical fiber and light source device - Google Patents

Description

  The present invention relates to an optical fiber and a light source device.

  When high-intensity light such as picosecond or femtosecond short pulse light, nanosecond pulse light, or continuous light propagates through a nonlinear medium such as an optical fiber, the band is expanded by the nonlinear optical phenomenon that appears in the nonlinear medium. It is known that the generated broadband light is generated. Broadband light obtained by this phenomenon is called supercontinuum (SC) light. Since SC light has the characteristics of wide band, high output, and flat spectrum, it is expected to be applied to optical measurement and near infrared spectroscopy.

  Non-Patent Document 1 describes a result of generating SC light by using a photonic crystal fiber (PCF) as an optical fiber that exhibits a nonlinear optical phenomenon. However, the PCF has a special structure in which a plurality of holes extending in the axial direction are formed in the cross section of the fiber, and there is a problem that the manufacturing cost is high.

  Therefore, many reports have been made that SC light is generated using a highly nonlinear optical fiber having a core with a high refractive index and a clad with a low refractive index based on silica glass (for example, Patent Documents 1 and 2). This highly nonlinear optical fiber is a solid one having no holes and has a simple structure.

  However, it has been difficult to extend the band of SC light to a long wavelength of 2400 nm or more and a short wavelength of 850 nm or less by the technique using the conventional highly nonlinear optical fiber. On the long wavelength side, when the wavelength is 2400 nm or more, the intensity of the seed light is attenuated by the infrared absorption of the quartz glass, and the nonlinear optical phenomenon cannot be exhibited.

  On the other hand, the shortest wavelength at which SC light is generated is determined by the wavelength at which the group refractive index matches the group refractive index at the longest wavelength of the SC generation band. The group index of light propagating in an optical fiber has a minimum at a certain wavelength (zero dispersion wavelength), and increases at both shorter and longer wavelengths. Therefore, in order to shorten the wavelength at which the longest wavelength determined by the infrared absorption of quartz glass and the group refractive index match and widen the band of SC light to the short wavelength side, the zero dispersion wavelength of the optical fiber should be as short as possible. It is effective to do. The zero dispersion wavelength of the optical fiber described in Patent Document 1 is 1350 to 1570 nm. The zero dispersion wavelength of the optical fiber described in Patent Document 2 is 1284 nm.

JP, 2007-279704, A JP, 2010-49089, A

W.J. Wadsworth, et al, "Supercontinuum and four-wave mixing with Q-switched pulses in endlessly single-mode photoniccrystal fibres," OPTICS EXPRESS, Vol.12, No.2, pp.299-309 (26 January 2004)

  In order to expand the band of the SC light on the short wavelength side, it is desired that the zero dispersion wavelength of the optical fiber be short, and it is shorter than the zero dispersion wavelength 1350 to 1570 nm of the optical fiber described in Patent Document 1. desired. The optical fiber described in Patent Document 2 has a zero dispersion wavelength shorter than the zero dispersion wavelength of the optical fiber described in Patent Document 1.

  However, in the optical fiber described in Patent Document 2, the core is pure silica glass, the mode field diameter at a wavelength of 1550 nm is as large as about 10 μm, and the nonlinearity is small. Therefore, this optical fiber cannot efficiently generate SC light, and needs a length of several tens of meters to 1 km.

  The present invention has been made to solve the above problems, and an optical fiber that has a shorter zero-dispersion wavelength and higher non-linearity than conventional ones and is capable of generating a wide band SC light with high efficiency, and It is an object of the present invention to provide a light source device that can output wideband SC light using an optical fiber.

The optical fiber of the present invention is made of quartz glass, has a zero dispersion wavelength of 1290 to 1350 nm, and has an effective cross-sectional area of 14 μm ² or less at a wavelength of 1550 nm.

  The optical fiber of the present invention has a shorter zero-dispersion wavelength and a higher non-linearity than conventional ones, and can generate broadband SC light with high efficiency.

FIG. 1 is a graph showing the wavelength dependence of the group refractive index of the optical fiber of the embodiment and the optical fiber of the comparative example. FIG. 2 is a diagram showing an example of a refractive index profile of the optical fiber of the embodiment. FIG. 3 is a graph showing the relationship between the core diameter 2a and the zero dispersion wavelength. FIG. 4 is a graph showing the relationship between the core diameter 2a and the effective area at a wavelength of 1550 nm. FIG. 5 is a graph showing the relationship between the core diameter 2a and the fiber cutoff wavelength. FIG. 6 is a graph showing the relationship between the relative refractive index difference Δ1 of the core and the zero dispersion wavelength. FIG. 7 is a graph showing the relationship between the relative refractive index difference Δ1 of the core and the effective area at a wavelength of 1550 nm. FIG. 8 is a graph showing the relationship between the relative refractive index difference Δ1 of the core and the fiber cutoff wavelength. FIG. 9 is a graph showing the relationship between b / a and zero dispersion wavelength. FIG. 10 is a graph showing the relationship between b / a and the effective area at a wavelength of 1550 nm. FIG. 11 is a graph showing the relationship between b / a and the fiber cutoff wavelength. FIG. 12 is a table in which specifications of the optical fibers of the comparative example and the optical fibers 1 to 16 of the example are summarized. FIG. 13 is a table summarizing the specifications of the optical fibers of the comparative example and the optical fibers 1 to 16 of the example. FIG. 14: is a figure which shows the structure of the light source device 1 of embodiment.

The optical fiber of the present invention is made of quartz glass, has a zero dispersion wavelength of 1290 to 1350 nm, and has an effective cross-sectional area of 14 μm ² or less at a wavelength of 1550 nm.

The fiber cutoff wavelength of this optical fiber is preferably 1650 to 2300 nm. The wavelength dispersion at a wavelength of 1550 nm is preferably 10 to 22 ps / nm / km. It is preferable that the nonlinear refractive index at a wavelength of 1550 nm is 6.0 × 10 ⁻²⁰ m ² / W or more.

This optical fiber has a core having a refractive index n1 and a diameter of 2a and including Ge, a depressed region surrounding the core and having a refractive index n2 and an outer diameter of 2b including F, and a depressed region surrounding the depressed region and having a refractive index n3. It is preferable to have a clad having the above and have a relationship of n1> n3 ≧ n2. The relative refractive index difference Δ1 of the core with respect to the depressed is 3.0 to 4.2%, the relative refractive index difference Δ2 of the depressed with respect to the clad is −0.8 to −0.3%, and 2a is 4. It is preferable that it is 0 to 6.0 μm and b / a is 2.0 to 3.0. Further, it is preferable that the relative refractive index difference Δclad of the clad with respect to pure SiO ₂ is −0.8 to 0%.

  The light source device of the present invention inputs a seed light source that outputs light having a center wavelength in the range of 1000 to 1650 nm, and propagates the light output from the seed light source by inputting and propagating the light. The optical fiber according to any one of claims 1 to 7, wherein the optical fiber generates and outputs broadband light whose bandwidth is expanded by an optical phenomenon.

  Hereinafter, a mode for carrying out the present invention will be described in detail with reference to the accompanying drawings. In the description of the drawings, the same elements will be denoted by the same reference symbols, without redundant description. The present invention is not limited to these examples, but is defined by the claims, and is intended to include meanings equivalent to the claims and all modifications within the scope.

  FIG. 1 is a graph showing the wavelength dependence of the group refractive index of the optical fiber of the embodiment and the optical fiber of the comparative example. The zero dispersion wavelength of the optical fiber of the embodiment is 1303 nm. The zero dispersion wavelength of the optical fiber of the comparative example is 1480 nm. As shown in this figure, in the optical fiber of the comparative example, the wavelength at which the group refractive index matches the group refractive index at the wavelength of 2400 nm is 1080 nm. In the optical fiber of the embodiment, the wavelength at which the group refractive index matches the group refractive index at the wavelength of 2400 nm is 760 nm. Therefore, the optical fiber of the embodiment can extend the band of SC light to the shorter wavelength side.

The optical fiber of the embodiment is made of silica glass, has a zero dispersion wavelength of 1290 to 1350 nm, and has an effective area Aeff at a wavelength of 1550 nm of 14 μm ² or less. This optical fiber can increase non-linearity, and can generate broadband SC light with high efficiency with a short fiber length. The non-linear coefficient γ [1 / W / km] that represents the non-linearity of the optical fiber is expressed by the following equation. Here, λ is a wavelength and n ₂ is a nonlinear refractive index [m ² / W]. n ₂ and γ are values measured by the XPM method in a linearly polarized state.
γ = 2πn ₂ / λAeff

The nonlinear refractive index n ₂ is preferably 6.0 × 10 ⁻²⁰ m ² / W or more. Γ at a wavelength of 1550 nm is preferably 20 [1 / W / km] or more, and more preferably 25 [1 / W / km] or more. The MFD at a wavelength of 1550 nm is preferably 4 μm or less, more preferably 3.6 μm or less.

  The fiber cutoff wavelength of the optical fiber of the embodiment is preferably 1650 to 2300 nm. In the optical fiber of the embodiment, by making the fiber cutoff wavelength longer than that of the conventional optical fiber, the propagation light can be more strongly confined in the core, and the influence of the waveguide dispersion on the chromatic dispersion can be reduced. As a result, the zero dispersion wavelength can be brought closer to the lower limit (1272 nm) of the zero dispersion wavelength determined by the glass material dispersion.

  When the fiber cutoff wavelength is long, the wavelength of the seed light may be shorter than the fiber cutoff wavelength, but by coupling the seed light mainly at the center of the optical fiber core at the incident end of the optical fiber, It is possible to avoid unnecessary high-order modes being excited at the entrance end. Moreover, since the difference in the propagation constant between the fundamental mode and the higher-order mode is large, it is possible to avoid coupling to an unnecessary higher-order mode while the light is propagating in the optical fiber. The seed light can be propagated by.

  The chromatic dispersion at the wavelength of 1550 nm of the optical fiber of the embodiment is preferably 10 to 22 ps / nm / km. By setting the wavelength dispersion in this range, the zero dispersion wavelength can be set to 1350 nm or less.

  FIG. 2 is a diagram showing an example of a refractive index profile of the optical fiber of the embodiment. The optical fiber of the embodiment is based on silica glass and has a core containing Ge and having a refractive index n1 and a diameter 2a, and a core including F having a refractive index n2 and an outer diameter 2b surrounding the core. It has a relationship of n1> n3 ≧ n2, which includes a prest and a clad that surrounds the depressed region and has a refractive index of n3.

The relative refractive index difference Δ1 [%] of the core with respect to the depressed. The depressed relative refractive index difference Δ2 [%] with respect to the cladding and the relative refractive index difference Δclad [%] of the cladding with respect to pure SiO ₂ (refractive index n0) are expressed by the following formulas.
Δ1 [%] = 100 × (n1-n2) / n1
Δ2 [%] = 100 × (n2-n3) / n2
Δclad [%] = 100 × (n3−n0) / n3

  FIG. 2A shows a case where n2 = n3. FIG. 2B shows a case where n2 <n3 = n0. FIG. 2C shows the case where n3 <n0, which can be realized by adding F to the clad. By setting n3 <n0 as shown in FIG. 2 (c), the amount of Ge added to the core for realizing the same Δ1 can be reduced as compared with FIG. 2 (b), and the transmission loss is reduced. can do. Further, by adding F to the clad, the viscosity of the clad, which occupies most of the volume of the optical fiber, can be reduced, so that the drawing temperature can be lowered and the transmission loss can be reduced. It is possible to add F that gives a relative refractive index difference of Δclad = −0.8%.

FIG. 3 is a graph showing the relationship between the core diameter 2a and the zero dispersion wavelength. FIG. 4 is a graph showing the relationship between the core diameter 2a and the effective area at a wavelength of 1550 nm. FIG. 5 is a graph showing the relationship between the core diameter 2a and the fiber cutoff wavelength. Here, Δ1 = 4.1%, Δ2 = −0.5%, Δclad = −0.3%, and b / a = 2.6.

From these figures, it is understood that the core diameter 2a is preferably 4.0 to 6.0 μm. From FIGS. 3 and 5, by setting the core diameter 2a to 4.0 μm or more, the zero dispersion wavelength can be set to 1350 nm or less, and the fiber cutoff wavelength can be set to 1650 nm or more. On the other hand, as shown in FIG. 4, when the core diameter 2a is 6.0 μm or more, the effective area Aeff is 14 μm ² or more, and the nonlinearity is deteriorated.

  FIG. 6 is a graph showing the relationship between the relative refractive index difference Δ1 of the core and the zero dispersion wavelength. FIG. 7 is a graph showing the relationship between the relative refractive index difference Δ1 of the core and the effective area at a wavelength of 1550 nm. FIG. 8 is a graph showing the relationship between the relative refractive index difference Δ1 of the core and the fiber cutoff wavelength. Here, b / a = 2.6 and 2a = 4.8 μm. In addition, (Δ2, Δclad) = (0%, −0.8%), (−0.5%, −0.3%), and (−0.8%, 0%) were used. In FIGS. 6 and 7, these three types of graphs almost overlap.

From these figures, it can be seen that by setting Δ1 to be 2.4% or more, the zero dispersion wavelength can be set to 1350 nm or less and the effective sectional area can be set to 14 μm ² or less. It is possible to add Ge to the core in an amount that raises the relative refractive index difference to pure SiO _{2 up} to about 3.4%, but it is practically difficult to add Ge at a higher concentration than that. Further, it is possible to add to the depressed amount an amount of F that lowers the relative refractive index difference with respect to pure SiO2 to about -0.8%, but it is practically difficult to add F at a higher concentration than that. is there. In reality, Δ1 has a practical upper limit of about 4.2%. Therefore, Δ1 is preferably 2.4 to 4.2%, more preferably 3.0 to 4.2%.

  Further, from these figures, it can be seen that even if Δ2 is changed, neither the zero dispersion wavelength nor the effective area is changed, and only the fiber cutoff wavelength is changed. Therefore, Δ2 may be selected so as to obtain a desired fiber cutoff wavelength. However, since it is practically difficult to add F in an amount equal to or more than the amount of F added corresponding to a relative refractive index difference of about −0.8% with respect to pure silica, Δ2 is practically −0.8. ˜0% is preferable, and −0.8 to −0.3% is more preferable.

  FIG. 9 is a graph showing the relationship between b / a and zero dispersion wavelength. FIG. 10 is a graph showing the relationship between b / a and the effective area at a wavelength of 1550 nm. FIG. 11 is a graph showing the relationship between b / a and the fiber cutoff wavelength. Here, Δ1 = 4.1%, Δ2 = −0.5%, Δclad = −0.3%, and a = 4.4 μm.

  From these figures, it is understood that the zero-dispersion wavelength becomes sharply long when b / a is 2.0 or less. On the other hand, increasing b / a to 3.0 or more has almost no effect on the characteristics. Therefore, b / a is preferably 2.0 to 3.0.

12 and 13 are tables summarizing specifications of the optical fiber of the comparative example and the optical fibers 1 to 16 of the example. FIG. 12 shows transmission loss at wavelength 1550 nm, zero dispersion wavelength, wavelength dispersion at wavelength 1550 nm, dispersion slope at wavelength 1550 nm, fiber cutoff wavelength, effective cross-sectional area at wavelength 1550 nm, mode field diameter at wavelength 1550 nm, and nonlinear refractive index n. ₂ and the nonlinear coefficient γ at a wavelength of 1550 nm are shown. In FIG. 13, Δ1, Δ2, Δclad, 2a and b / a are shown. The α-th power representing the shape of the refractive index distribution of the core of the optical fiber of each of the examples is about 1.6 to 3.0 power.

  FIG. 14: is a figure which shows the structure of the light source device 1 of embodiment. The light source device 1 includes the optical fiber 10 of the above embodiment and the seed light source 20. The seed light source 20 outputs seed light having a center wavelength in the range of 1000 to 1650 nm. More preferably, the seed light source 20 outputs seed light having a center wavelength in the range of 1200 to 1400 nm. The seed light output from the seed light source 20 is high intensity light such as picosecond or femtosecond short pulse light, nanosecond pulse light, or continuous light. The optical fiber 10 inputs the seed light output from the seed light source 20 to one end thereof and propagates the seed light to generate wide band light (SC light) whose band is expanded by a non-linear optical phenomenon that appears during the propagation of this seed light. Then, the SC light is output from the other end. The light source device 1 can output SC light in a wider band than conventional ones.

  1 ... Light source device, 10 ... Optical fiber, 20 ... Seed light source.

Claims (6)

Made of quartz glass,
A core including Ge having a refractive index n1 and a diameter 2a; a depressed region surrounding the core, including a refractive index n2 and an outer diameter 2b including F; and a clad surrounding the depressed region and having a refractive index n3. , N1>n3> n2,
The zero dispersion wavelength is 1290 to 1350 nm,
Ri Der effective area 14 [mu] m ² or less at a wavelength of 1550 nm,
The fiber cutoff wavelength is greater than 1650 nm and 2300 nm or less,
Optical fiber. The wavelength dispersion at a wavelength of 1550 nm is 10 to 22 ps / nm / km,
The optical fiber according to claim 1 . The nonlinear refractive index at a wavelength of 1550 nm is 6.0 × 10 ⁻²⁰ m ² / W or more,
The optical fiber according to claim 1 . The relative refractive index difference Δ1 of the core with respect to depressed is 3.0 to 4.2%,
The relative refractive index difference Δ2 of the depressed with respect to the clad is −0.8 to −0.3%,
2a is 4.0 to 6.0 μm,
b / a is 2.0 to 3.0,
The optical fiber according to claim 1. The relative refractive index difference Δclad of the clad with respect to pure SiO ₂ is −0.8 to 0%,
The optical fiber according to any one of claims 1 to 4 . A seed light source that outputs light having a center wavelength in the range of 1000 to 1650 nm;
Is propagated by inputting the light output from the seed light source, any one of claim 1 to 5 to generate broadband light band is expanded output by a nonlinear optical phenomenon that expression during propagation of the light An optical fiber described in
A light source device.

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